The present invention relates to a control apparatus for controlling a variable speed motor, and more specifically to a control apparatus for controlling a variable speed motor that suppresses mechanical vibration in a system consisting of a load coupled to the variable speed motor being driven at variable speeds.
As seen in FIG. 33, a block diagram shows a circuit configuration of a prior art control apparatus for controlling a variable speed motor. In FIG. 33, a load 4 is coupled to a motor 2 through an elastic joint 3, a power converter 5 supplies electric power to the motor 2, and a controller 9 controls the power converter 5. The motor 2 drives the load 4 at a rotation speed that coincides with a target speed value set by a speed setting device 8 in response to a control signal that the controller 9 generates. The controller 9 consists of a speed regulator 12, a current regulator 14, a fifth weighting adder 103, and a third state observer 104.
Since the load 4 is coupled to the motor 2 through the elastic joint 3, a relationship among motor speed .omega..sub.M, load speed .omega..sub.L and axial torque .tau..sub.S is expressed by following equations 1, 2 and 3, in which J.sub.M is moment of inertia of the motor 2, J.sub.L is moment of inertia of the load 4, K.sub.S is a spring constant of the elastic joint 3, .tau..sub.L is load torque, .tau..sub.A is drive torque and s is a Laplace operator. ##EQU1## When the load torque .tau..sub.L is zero, the motor speed .omega..sub.M is expressed by a following equation 4. ##EQU2##
One can ascertain from the polynomial denominator of equation 4 that the mechanical system has a resonance point. In equation 4, a controllable variable is the drive torque .tau..sub.A which is expressed by a following equation 5, in which K.sub.T is a coefficient and I.sub.T is an armature current value (torque component). EQU .tau..sub.A =K.sub.T .multidot.I.sub.T ( 5)
Still referring to FIG. 33, a current detector 6 and a speed detector 7 detect an armature current value of the motor 2 and the speed of the motor 2, respectively. The third state observer 104, to which the detected armature current value, which generates the drive torque, and speed of the motor 2 are sent, estimates state variables such as the load speed .omega..sub.L, load torque .tau..sub.L, and axial torque .tau..sub.S which can not be detected directly. In addition, the motor speed detected by the speed detector 7 and a target speed set by a speed setting device 8 are supplied to the speed regulator 12. The speed regulator 12 executes control operations based on the supplied detected motor speed and target speed, and the speed regulator outputs a target current value calculated to reduce deviation of the detected motor speed from the target speed to zero.
The fifth weighting adder 103 calculates a weighted sum of the state variables supplied from the third state observer 104 and the target speed supplied from the speed regulator 12, and outputs the resultant weighted sum as a new target current value to the current regulator 14 to control the drive torque.
Turning to the current detector 6, it detects the armature current that flows through the armature of the motor 2. The detected armature current value and the target current value described above are fed to the current regulator 14. The current regulator 14 transmits to a power converter 5 a target voltage value calculated to reduce deviation between the detected armature current value and the target current value to zero. The power converter 5 in turn drives the motor 2 in response to the target voltage value set by the current regulator 14.
In FIG. 33, the third state observer 104 estimates the aforementioned state variables on the basis of the detected current value and detected motor speed, and outputs the estimated state variables to the current regulator 14 to control the drive torque and to suppress mechanical vibrations. However, because the speed detector 7 cuts off high frequency signals, mechanical vibrations of frequencies higher than the cut-off frequency of the speed detector 7 can not be suppressed by utilizing the third state observer 104. Therefore, the variable speed motor controller of FIG. 33 cannot stably control the motor 2 when vibration frequency exceeds the cut-off frequency of the speed detector 7.
In view of the foregoing, it is an object of the present invention to provide a control apparatus for controlling mechanical vibrations of high frequencies in variable speed motors.
It is another object of the present invention to provide a control apparatus for suppressing mechanical vibrations in variable speed motors, which control apparatus does not require the use of a motor-speed detector and controls the variable speed motor stably.
It is yet another object of the present invention to provide a control apparatus for suppressing mechanical vibrations in variable speed motors, which control apparatus utilizes motor-speed value detected by a motor-speed detector only for vibration frequencies which are lower than the cut-off frequency of the motor speed detector, and which control apparatus controls the variable speed motor stably.